Attributes named in this macro are protected from mass-assignment whenever
attributes are sanitized before assignment. A role for the attributes is
optional, if no role is provided then :default is used. A role can be
defined by using the :as option.

Mass-assignment to these attributes will simply be ignored, to assign to
them you can use direct writer methods. This is meant to protect sensitive
attributes from being overwritten by malicious users tampering with URLs or
forms. Example: